Example fixes about nanodbc

[lexicalunit]

Unlike C, the return statement in main() will be created for you in C++, default return 0, if left unspecified. Some C++ devs like to leave it in, but I usually omit it since it's not strictly needed, and most of the C++ code I see tends to do that too.

C++ Standard § 3.6.1 paragraph 5: "A return statement in main has the effect of leaving the main function (destroying any objects with automatic storage duration) and calling std::exit with the return value as the argument. If control reaches the end of main without encountering a return statement, the effect is that of executing return 0;"

The usage info is fantastic, much thanks!